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in  2012  with  funding  from 

University  of  Illinois  Urbana-Champaign 


http://archive.org/details/storyofgeologicm01well 


Tower  Rock,  Hardin  County 


m  * 


STATE    OF    ILLINOIS 
DEPARTMENT    OF    REGISTRATION    AND    EDUCATION 

M.  F.  Walsh,  Director 

DIVISION   OF  THE 

STATE  GEOLOGICAL   SURVEY 


M.    M.   LEIGHTON,    Chief 


EDUCATIONAL  SERIES  NO.  1 


The  Story  of  the  Geologic  Making 
of  Southern  Illinois 

BY 

STUART  WELLER 


Printed  by  authority  of  the  State  of  Illinois 


URBAN  A,   ILLINOIS 

1927 

Reprinted  1031 


Department  of  Registration  and  Education 

State  Board  of  Natural  Resources  and  Conservation 

Committee  on  Geological  Survey 

M.  F.  Walsh,  Chairman 

Director  of  Registration  and  Education 

Charles  M.  Thompson 

Representing  the  President  of  the  Uni- 
versity of  Illinois 

Edson  S.  Bastin 

Geologist  ? 


Jeffersons  Printing  &  Stationery  Co. 

Springfield,  Illinois 

1931 


Letter  of  Transmittal 


State  Geological  Survey  Division,  Urbana,  May  27,  1931 

M.  F.   Walsh,  Chairman ,  and  Me  tubers  of  the  Board  of 
Natural  Resources  and   Conservation , 

Gentlemen: 

The  first  edition  of  Education  Series  No.  1,  "The  Story 
of  the  Geologic  Making  of  Southern  Illinois,"  by  the  late 
Professor  Stuart  Weller,  has  become  exhausted  and  a  second 
edition  is  necessary  to  meet  the  demands  of  our  public  schools 
and  of  our  laymen  for  this  popular  treatment.  I  therefore 
respectfully  recommend  its  reprinting. 

Sincerely  yours, 

M.  M.  Leighton,  Chief. 


Preface 

In  southern  Illinois  lies  a  region  which  occupies  about  one 
twenty-eighth  of  the  area  of  the  State  and  which  differs  marked- 
ly in  appearance  from  the  rest  of  the  State.  By  far  the  greatest 
portion  of  Illinois  is  made  up  of  flat,  fertile  prairies;  indeed, 
in  the  central  section,  hills  are  exceedingly  scarce.  A  traveler 
making  his  way  south  from  Harrisburg  in  Saline  County  finds 
that  he  has  left  the  prairies  behind  and  that  he  is  among  hills, 
the  highest  one  of  which — Williams  Hill  in  Pope  County — 
attains  an  elevation  of   1,065   feet.     In  no  other  part  of  the 


CAVE  IN-ROCK 
ROSICLARE 


|5    MILCS 


Fig.   1. — Map  of  southern  Illinois   showing  approximate  boundaries 
of  Ozark  Ridsre. 


State  save  in  Jo  Daviess  County  in  the  northwest  corner  are 
there  higher  hills.  The  traveler  finds  short,  swift  streams  and 
steep  ascents,  hidden  nooks  and  outlook  points,  and  a  relief 
made  most  engaging  to  the  eye  In    reason  of  rapidl)   changing 


scenes.  And  nowhere  does  he  find  the  extensive  fields  and 
farms  so  characteristic  of  the  prairies. 

This  area  of  southern  Illinois  covers  a  strip  about  70  miles 
long  and  from  15  to  40  miles  wide  (fig.  1).  It  extends  from 
Mississippi  River  on  the  west  to  Ohio  River  on  the  east,  and 
lowlands  lie  both  to  the  north  and  to  the  south  of  it. 

The  history  which  follows  attempts  to  outline  briefly  the 
manner  in  which  southern  Illinois  came  to  be  as  we  know  it. 
Much  detail  necessarily  has  been  omitted,  and  much  remains  to 
be  learned,  but  not  a  year  passes  without  the  acquisition  of 
some  new  information  which  helps  to  complete  the  story. 


THE  STORY  OF  THE  GEOLOGIC 
MAKING  OF  SOUTHERN  ILLINOIS* 

By    Stuart   Welter 
INTRODUCTION 

When  man  began  to  wonder  about  the  reasons  for  things 
and  began  to  seek  out  the  causes  for  the  great  variety  of  natural 
phenomena  which  surrounded  him,  he  began  the  ascent  of  the 
intellectual  ladder  many  of  whose  steps  he  has  already  climbed, 
but  from  the  top  of  which  he  is  still  much  farther  removed  than 
from  the  bottom.  In  the  early  stages  of  this  intellectual  growth 
he  had  no  perception  of  natural  law  and  he  of  necessity  had  to 
explain  the  phenomena  of  nature  through  supernatural  agencies. 
Thus  grew  up  the  myths  of  creation  as  well  as  other  myths  and 
stories  to  account  for  all  of  the  unexplainable  phenomena  about 
him.  Many,  many  generations  passed  before  man's  intelligence 
began  to  give  him  some  conception  of  the  workings  of  the  laws 
of  nature.  Undoubtedly,  from  time  to  time  some  great  genius 
arose  whose  sense  of  curiosity  was  more  intense  than  that  of 
his  fellows, — one  who  was  capable  of  searching  deeply  into  the 
reasons  for  things  and  who  was  able  to  make  some  addition  to 
the  knowledge  of  the  human  race.  Comparatively  few  genera- 
tions of  men  have  passed  since  Newton  formulated  the  law  of 
gravitation,  perhaps  the  most  far-reaching  of  all  natural  laws. 
Copernicus,  who  founded  all  modern  astronomy  by  his  dis- 
covery that  the  earth  was  not  stationary  but  revolved  about 
the  sun,  was  born  less  than  500  years  ago.  Even  today  the 
vast  majority  of  human  beings,  and  among  them  great  numbers 
of  our  own  people,  have  but  little  conception  of  the  meaning  of 

*  This  paper  is  based  on  an  address  given  by  the  author  as  retir- 
ing- president  of  the  Illinois  State  Academy  of  Science  at  the  Harris- 
burg  meetings,  April  30,  1926,  and  published  in  the  Proceedings  of  the 
Academy,  vol.  19,  pp.  27-49. 


FEATURES    OK   THE    LANDSCAPE 


science,  and  still  hold  to  all  sorts  of  supernatural  explanations 
of  phenomena  which  they  observe  every  day. 

Reduced  to  its  last  analysis,  science  is  the  search  for  the 
reasons  for  things.  Every  man  of  science  knows  that  every 
phenomenon  in  nature  has  a  cause,  a  reason,  and  just  as  truly 
every  phenomenon  has  an  effect.  The  search  for  the  causal 
relations  of  all  these  phenomena  is  his  task.  If  he  is  unable 
to  discover  a  reason  for  the  facts  which  he  observes,  he  does 
not  appeal  to  some  supernatural  explanation ;  he  knows  that  his 
researches  have  not  yet  penetrated  far  enough  into  his  problem. 
The  men  of  science  of  our  generation  are  those  in  whom  the 
sense  of  curiosity  concerning  nature  is  most  keenly  developed. 
They  are  continually  asking  themselves  the  question,  "Why?" 
and  are  never  satisfied  until  some  reasonable  and  logical  ex- 
planation is  found.  Every  child  starts  his  career  as  an  active 
researcher.  He  observes  all  sorts  of  phenomena  about  him 
which  arouse  his  curiosity,  and  he  repeatedly  asks  the  question 
"Why?"  All  too  often  his  questioning  soon  surpasses  the  ability 
of  his  parents  and  teachers  to  make  answer  and  he  is  told  not 
to  ask  such  foolish  questions.  Doubtless  many  an  able  man 
of  science  has  been  lost  through  the  stifling  of  childish  question- 
ing. Fortunately  a  few  remain  in  whom  this  curiosity  cannot 
be  stifled,  else  the  progress  of  our  race  would  come  to  a  stand- 
still. 

EARLY  CONCEPTIONS 

Among  the  phenomena  which  early  attracted  the  attention 

of  man,  were  the  features  of  the  landscape  which  surrounded 

him,  the  hills  and  valleys,  the  mountains  and  the  sea.  Because 

„  r      of  their  apparent  stability,  explanations  for  these 

Features  of  ,  . 

the  phenomena  were  not   sought   as   early   as   were 

landscape  explanations     for    the     apparent    movement    of 

heavenly    bodies.      If    any    thought    was    given    to    the    sub- 
ject,   it  was   assumed   that   the   hills   and   valleys   and    moun- 


GEOJ  OGIC  TIME  AND  PROCESSES  I  I 

tains  and  seas  had  always  been  and  would  always  be.  This 
conception  of  permanence  is  still  with  us  and  not  infrequently 
we  see  references  to  the  everlasting  hills,  although  every  geolo- 
gist knows  that  neither  the  hills  nor  the  highest  mountains  are 
everlasting. 

Another  group  of  objects  which  came  to  the  attention  of 
early  observers  were  the  fossil  shells  seen  in  all  sorts  of  situ- 
tions.  During  the  Middle  Ages  fossils  aroused  much  discussion 
and  numerous  explanations  were  offered  for 
them.  Some  thought  them  to  be  sports  of 
nature;  some  believed  them  to  be  discards  thrown  aside  by 
the  Creator  of  all  things ;  but  the  final  explanation  which 
seemed  to  satisfy  most  men  was  that  they  were  the  remains 
of  sea  animals  which  were  swept  from  the  sea  and  were  lodged 
upon  the  land  during  the  Noachian  Deluge.  Of  course  the 
geologist  today  knows  that  these  objects  are  the  remains  of 
animals  and  plants,  many  of  them  marine,  which  one  time 
lived  and  flourished  in  the  places  in  which  the  evidences  of 
their  existence  are  now  found.  At  that  time  the  ocean  waters 
covered  those  areas  now  dry  land  but  formerly  parts  of  the 
sea  bottom. 

GEOLOGIC  TIME  AND  PROCESSES 

It  was  only  a  little  more  than  a  hundred  years  ago  that 
students  of  geology  began  to  realize  that  the  present  configura- 
tion of  the  earth's  surface  has  been  established  through  the 
agency  of  slowly  acting  processes  which  are  in  operation  today 
and  which  have  been  continuously  in  operation  for  long  periods 
of  time,  rather  than  through  abrupt,  cataclysmic  revolutions 
which  occurred  at  comparatively  short  intervals.  We  have  come 
to  realize  that  geologic  time  is  long — exceedingly  long  when 
measured  by  the  scale  of  human  existence — and  we  look  upon 
its   duration    as   being   comparable,    in    a    degree,    to    the   vast 


12  GEOLOGIC   TIME    AND    PROCESSES 

extensions  of  space  recognized  by  the  astronomer.  And  with  an 
increase  in  our  knowledge  of  the  earth's  history  comes  an  ex- 
pansion in  our  conception  of  geologic  time.  In  the  history  of 
the  earth  a  million  years  are  but  as  a  day,  and  forces  and 
processes  which  seem  to  have  only  a  small  effect  when  seen  from 
day  to  day  are  capable  of  producing  tremendous 

Modern  effects    when    continued    over    long    periods    of 

conceptions        .  .  ° 

time.     During  a  summer  shower,  all  of  us  have 

seen  material  transferred  from  the  hilltops  to  the  valleys  and 
if  enough  time  is  allowed  and  enough  showers  fall  upon  a  hill, 
eventually  that  hill  will  be  leveled  to  the  plains.  In  the  geo- 
logic past  this  very  transfer  of  material  has  taken  place  repeat- 
edly. Mountains  as  high  as  the  highest  upon  earth  today  have 
been  eroded  down  to  the  level  of  the  sea;  other  mountains 
have  been  raised  up  and  have  in  turn  been  worn  away. 

The  geologist  interprets  the  history  of  the  earth's  past  re- 
corded in  the  rock  strata  of  its  crust  through  an  understanding 
of  the  processes  which  are  active  today.  It  goes  without  saying 
that  in  the  erection  of  a  pile  of  lumber  the  boards  in  the  bottom 
of  the  pile  were  laid  down  first.  Likewise  the  lowermost  beds 
in  the  rock  strata  of  the  earth's  crust  in  any  given  region  were, 
axiomatically,  laid  down  first  and  consequently  are  the  oldest. 
Figure  2  shows  horizontal  beds  of  limestone.  In  order  for  the 
lowermost  bed  to  have  its  position  with  respect  to  the  other 
beds  above  it,  it  had  to  be  laid  down  first.  Hence,  it  is  the  old- 
est bed,  and  the  beds  above  it  are  successively  younger. 

If  the  solid  rock  formations  were  everywhere  uncovered 
and  had  never  been  disturbed,  it  would  be  a  comparatively 
simple  matter  to  trace  the  beds  from  place  to  place  and  estab- 
lish their  true  relations.  In  most  regions,  how- 
Correlation  ever>  these  conditions  do  not  exist.  The  rocks 
are  covered  in  many  places  by  unconsolidated 
residuum   and   soil.      In   parts   of   Illinois   whole   counties   are 


CORRELATION    OF    ROCKS 


13 


without  a  single  solid  rock  outcrop;  in  many  others  the  out- 
crops are  so  few  and  so  widely  separated  that  the  beds  cannot 
be  traced.  Then  again,  through  the  agency  of  great  deforma- 
tive  processes  the  rocks  have  been  broken  into  blocks  which 
have  been  shifted  up  and  down  and  horizontally  to  such  an 
extent  that  the  individual  beds  are  no  longer  continuous. 

Under  these  conditions  it  has  been  necessary  for  the  geol- 
ogist to  establish  some  means  of  tracing  rock  beds  from  place 


Fig.  2. — Exposure  of  limestone  beds  in  Kankakee  County,   Illinois. 


to  place,  and  long  study  has  shown  that  the  only  reliable 
criteria  are  the  organic  fossils  contained  in  the  rocks. 
These  fossils  furnish  a  record  of  the  evolution  of  the  life 
of  our  globe,  the  scattered  remains  of  succeeding  generations 
which  exhibit  the  gradual  changes  through  which  living  crea- 
tures have  been  passing.  The  life  of  no  period  of  geologic  time 
is  the  same  as  that  of  the  preceding  or  of  the  succeeding  period  ; 


14  GEOLOGIC  TIME  AND  PROCESSES 

consequently  a  knowledge  of  the  succession  of  living  things  upon 
the  earth  affords  us  a  most  reliable  criterion  for  the  determina- 
tion of  the  relative  ages  of  the  rock  strata  which  come  under 
our  observation.  It  must  be  recognized  that  our  knowledge  of 
these  long-extinct  inhabitants  of  the  earth  is  as  yet  but  frag- 
mentary, but  every  year  adds  to  our  information,  and  already 
we  can  make  a  serviceable  application  of  accumulated  facts  in 
our  interpretation  of  geologic  history.  With  further  informa- 
tion greater  and  greater  refinements  in  our  interpretations  will 
become  possible. 

If  the  earth's  crust  had  undergone  no  deformative  changes 
during  its  long  period  of  existence,  all  sediments  which  were 
accumulated  under  marine  waters  and  in  which  are  buried  the 

.,  remains  of  marine  organisms,  would  still  be  sub- 

Movements  .  .  .    .  ^  111-  Ml 

in  earth's  marine  in  position  and  would  be  inaccessible  for 

crust  study.     It  is  known,  however,  that  crustal  move- 

ments have  taken  place  even  in  historical  time.  Certain 
portions  of  our  shorelines  are  known  to  be  rising  relative 
to  the  ocean  level  and  other  portions  are  sinking.  Not  in- 
frequently violent  earthquakes  occur  in  the  course  of  which 
notable  crustal  changes  take  place.  Rocks  which  are  mani- 
festly of  marine  origin  since  they  contain  fossil  shells  of  marine 
organisms,  are  now  found  thousands  of  feet  above  sea  level; 
they  must  have  been  elevated  to  their  present  position  or  the  sea 
must  have  receded.  Probably  both  elevation  and  recession 
occurred.  Rocks  which  must  have  been  formed  originally  as 
nearly  flat-lying  sediments  deposited  in  water,  are  now  steeply 
inclined  in  many  places,  and  are  even  folded,  crushed  and 
broken.  The  existence  of  tremendous  forces  which  have  been 
at  work  in  earth-growing  processes  is  thus  made  manifest. 

No  sooner  was  land  exposed  above  the  level  of  the  sea  than 
it  was  subjected  to  the  processes  of  erosion.  Water  falling  as 
rain  and  always  seeking  a  lower  level  started  to  wear  away  the 


DIVISION    OF    GEOLOGIC    TIME  15 

^      .  elevated  rocks  and  gradually  wore  them  down  to 

Erosion  .  . 

of  earth's         sea  level,  just  as  the  hills  between   Harrisburg 

surface  an(j  Ohio  River  are  at  present  being  gradually 

reduced  by  every  rain  which  falls  upon  them. 

Deformation  and  unequal  erosion  have  resulted  in  the 
exposure  somewhere  at  the  surface  of  the  earth  of  sedimentary 
rocks  of  all  ages.  In  future  geologic  periods,  rocks  which  now 
exist  at  the  surface  will  have  been  worn  away  and  their  mate- 
rials will  have  been  carried  down  the  streams  to  the  ocean  and 
redeposited  to  form  younger  rocks,  much  as  sediments  are  now 
accumulating  in  the  delta  of  Mississippi  River. 

Our  present  knowledge   of   the  earth   has  progressed   far 
enough  to  make  it  possible  for  geologists  to  recognize  a  number 
of  distinct  divisions  of  geologic  time.     Of  course  time  is  con- 
tinuous and  uninterrupted,  and  we  can  divide  it 

Division  of       -nt0  d^tinct  periods  only  on  the  basis  of  events, 
geologic  time  .....  .         . 

Our  subdivision  of   time   in   human  history  has 

been  wholly  dependent  upon  the  occurrence  of  events.  The 
discovery  of  a  new  continent  in  the  fifteenth  century  ini- 
tiated into  human  history  a  new  period  of  the  utmost 
importance.  The  events  which  led  to  the  declaration  of 
war  in  Europe  in  1914  ushered  in  a  period  which  has  changed 
and  is  changing  the  destinies  of  most  of  the  nations  of  the  world. 
The  transition  of  one  period  into  another  may  be  gradual  like 
the  passing  of  spring  and  the  coming  of  summer,  or  it  may  be 
abrupt  and  unexpected  like  the  beginning  of  the  great  World 
War.  The  periods  of  geologic  history  are  not  unlike  those  of 
human  history.  They  are  definitely  marked  from  one  another 
by  events  like  the  growth  of  great  mountain  ranges,  or  the 
appearance  of  a  new  type  of  life.  Some  transitions  have  been 
abrupt  and  have  been  accomplished  in  an  exceedingly  short 
time,    geologically   speaking;   others   have   been    so   slow   as   to 


16  GEOLOGIC  TIME  AND  PROCESSES 

make  difficult  the  setting  of  a  precise  boundary  between  two 
rock  systems  although  each  has  distinct  characteristics. 

It  has  come  to  be  the  custom  of  geologists  to  use  as  their 

basis  for  the  larger  divisions  of  earth  history  the  character  of 

the  life  of  the  era.     Azoic  time  was  an  era  during  which  no 

life  existed.     Eozoic  was  the  era  of  the  dawn  of 

Eras  of     ^         life  with  the  introduction  of  unicellular  organ- 
geologic  time    .  r 
isms.       1  he    Archeozoic    era    was    the    age    01 

larval  life,  and  the  Proterozoic  the  age  of  primitive  inverte- 
brates. All  earth  history  to  the  close  of  Proterozoic  time 
is  so  ancient  that  our  information  regarding  it  is  most  hazy. 
The  Proterozoic  was  followed  by  eras  concerning  which  our 
information  is  less  limited  although  it  is  far  from  complete. 
The  Paleozoic  era  was  the  time  of  ancient  life  forms,  now 
mostly  extinct,  but  some  of  them  were  more  or  less  related  to 
our  contemporaneous  life.  The  Mesozoic  era  was  the  time  of 
medieval  life  which,  although  almost  wholly  unlike  living  forms 
of  today,  still  had  much  more  in  common  with  them  than  had 
the  life  of  the  Paleozoic.  The  Cenozoic  era  was  the  time  of 
modern  life  which  was  much  like  that  of  today.  The  last,  or 
Psychozoic,  era  is  the  age  of  man. 

Each  one  of  these  great  eras  is  divided  into  periods  some 

of  which  have  been  named  geographically  after  some  district 

in  which  rocks  representing  the  period  are  well  exhibited.  One 

great    period    of    the    Paleozoic    era    has    been 

Periods  of  named  the  Devonian  because  it  was  first  studied 
geologic  time 

and  its  fossil  life  was  first  described  at  Devon- 
shire, England.  Following  the  Devonian  wras  a  great  period 
known  as  the  Mississippian,  so  named  because  its  rocks  are 
best  represented  in  the  Mississippi  Valley  region  of  this  coun- 
try. The  Mississippian  was  succeeded  by  a  period  during 
which  the  great  coal  formations  of  eastern  North  America  were 


DIVISION    OF    GEOLOGIC   TIME 


17 


laid  down.    The  name  Pennsylvanian  has  been  given  it  because 
the  rocks  are  widely  developed  in  Pennsylvania. 

Table  1. — Table  of  geologic  time  divisions. 


Era 

Period  (time  term)  or 
System   (rock  term) 

Character  of  rocks  in 
Southern  Illinois 

Cenozoic 

"Recent   life" 

(Age   of   mammals) 

Recent 

Pleistocene 

Tertiary 

Surficial  deposits 
Absent 

Mesozoic 

"Middle  life" 

(Age  of  reptiles) 

Cretaceous 
Comanchean 
Jurassic 
Triassic 

Absent 

Paleozoic 
"Ancient  life" 
(Era  of  invertebrate 
animals    and    non- 
flowering   plants) 

Permian 
Pennsylvanian 
(Coal  Measures) 
Upper 
Mississippian 

Lower 
Devonian 
Silurian 
Ordovician 
Cambrian 

Absent 
Conglomerates,   sandstones, 

shales,  and  coal 
Sandstones,   shales,   and 

limestones 
Principally  limestones 
Limestone  and  shale 
Limestone 

Limestone  and  sandstone 
Not  exposed 

Proterozoic 
"Primitive  life" 

Archeozoic 
"Larval    life" 

Table  1  gives  the  eras  and  periods  of  geologic  time.  As  the 
Archeozoic  era  is  the  oldest  that  we  know  and  its  rocks 
were  formed  first,  it  is  placed  at  the  bottom  of  the  list. 
Similarly,  the  Cenozoic  era  is  the  youngest  and  is  placed  at 
the  top  of  the  column.  More  detailed  information  regarding 
the  geologic  time  table  and  geologic  principles  can  be  found  in 
the  standard  text  books  on  geology. 


18 


ROCKS  OF  SOUTHERN  ILLINOIS 


ROCKS  OF  SOUTHERN  ILLINOIS 

By  reason  of  the  processes  which  have  been  described  all 
too  briefly,  rocks  belonging  to  the  Ordovician,  Silurian,  Devon- 
ian, Mississippian,  and  Pennsylvanian  periods  are  now  exposed 
at  the  surface  in  southern  Illinois. 

The  Ordovician  and  Silurian  rocks  are  exposed  in  the 
Mississippi  River  bluffs  in  western  Alexander  County.  Devon- 
ian rocks  are  found  in  two  areas;  (1)  along  Mississippi 
River  in  Alexander  County,  Union  County,  and  a  very  small 
part  of  Jackson  County;  and  (2)  in  Hardin  County. 


Fig.  3. — Diagram  to  illustrate  effect  on  horizontal  beds  of  intru- 
sion of  molten  rock  or  lava  (A). 


The  second  area  of  Devonian  outcrop  is  limited  and  is 
known  to  geologists  as  Hicks  Dome.  It  is  a  structure  which  is 
comparable  to  a  doming  of  the  earth's  crust.  Figure  3  illustrates 
the  probable  manner  of  formation  of  this  dome. 
The  rock  strata  were  at  one  time  nearly  hori- 
zontal.    A  mass  of  material   (A  in  the  diagram),  probably  in 


Hicks  Dome 


DEVONIAN    AND   OLDER   ROCKS  19 

the  form  of  molten  rock  or  lava,  was  injected  into  the  crust 
beneath  the  horizontal  strata.  This  mass  pushed  up  the  beds 
above  it  and  a  doming  at  the  surface  resulted.  The  molten 
material  is  now  completely  cooled  and  solidified.  After  the  up- 
lifting, erosion  began  on  the  higher  parts  of  the  dome  and 
the  overlying  strata  were  worn  away  until  now  the  lower  and 
older  beds  are  exposed  in  the  center  of  the  dome.  Figure  4 
is  a  geologic  cross-section  of  Hicks  Dome,  in  which  the  vertical 
scale  has  been  much  exaggerated. 

Wherever  a  deep  drill-hole  is  sunk  for  oil  or  for  other 
purposes,  the  geologist  can  make  logical  deductions  from  the 
information    which    it    gives    regarding    the    rocks    penetrated. 

~         .  So  we   know  that  the   Devonian   rocks  continue 

Devonian 

and  older  beneath  the  surface  from  the  area  of  exposure  in 

rocks  Hardin    County,    across    southern    Illinois    into 

Union  County  where  they  are  again  exposed,  and  into  south- 
Hicks  Dome 


Fig.  4. — Geologic  cross-section   of   Hicks  Dome,   Hardin   County. 

eastern  Missouri.  To  the  east  and  south  they  are  com- 
pletely hidden  by  younger,  overlying  strata.  Of  course  the 
Devonian  rocks  are  not  the  oldest  rocks  in  southern  Illinois. 
We  know  about  the  rocks  and  their  attitudes  in  other 
parts  of  the  Mississippi  and  Ohio  valleys  and,  making  logical 
deductions,  we  know  that  beneath  the  Devonian  rocks  of 
Hardin  County  are  formations  representative  of  all  the  major 
geological  systems  below  the  Devonian.  Some  of  these  older 
rocks,  as  stated  above,  are  exposed  in  the  Mississippi  River 
bluffs. 


20 


ROCKS  OF  SOUTHERN  ILLINOIS 


The  southern  slope  of  the  Ozark  hills  south  of  Harris- 
burg  is  underlain  by  rocks  of  Mississippian  age;  the  higher 
parts  are  composed  of  Pennsylvanian  strata.     These  Pennsyl- 

Mississippian  Vanfan  r0cks  dip  t0  the  north'  and  at  Harris" 
and  Pennsyl-  burg  the  strata  which  are  exposed  at  the  tops 
vanian  rocks     of    the    Qzark   hiUs   are   found    faf   beneath    the 

surface.  Figure  5  shows  this  condition  in  diagrammatic  form. 
The    coal    beds   which    are    so    extensively   mined    in    southern 


Norfli 


South 


Fig.  5. — Diagram  illustrating  dip  of  rocks  south  of  Harrisburg. 
Vertical  scale  much  exaggerated.  Bed  A  outcrops  at  the 
surface  at  the  top  of  the  hill  south  of  Rudement,  but  at  Har- 
risburg it  is  far  beneath  the  surface. 


Illinois  belong  stratigraphically  far  above  the  sandstone 
which  forms  the  tops  of  the  Ozark  hills.  When  they  were 
originally  deposited,  the  coal  beds  were  undoubtedly  continu- 
ous with  the  existing  coal  fields  of  western  Kentucky,  but  sub- 
sequently an  area  in  Hardin  County  and  adjacent  portions  of 
Kentucky  was  uplifted  and  not  less  that  3,000  feet  of  hard  rock 
strata  have  been  removed  by  the  slow  processes  of  erosion.  The 
Ohio  River  has  cut  its  valley  through  all  these  strata. 


DEVONIAN   SEA  21 


HISTORY  OF  THE  MAKING  OF 
SOUTHERN  ILLINOIS 

No  history  of  the  making  of  southern  Illinois  can  be  com- 
plete without  taking  into  account  those  rocks  older  than  the 
Devonian,  but  as  the  major  events  which  led  to  the  present 
configuration  occurred  after  this  period,  and  as  in  only  a  small 
area  are  older  rocks  exposed,  we  shall  begin  our  story  with 
Devonian  conditions. 

The  oldest  Devonian  rocks  are  limestones  in  which  are 
buried  numerous  fossil  shells  of  marine  animals,  and  we  can 
confidently  assert  that  during  Devonian  times  southern  Illinois 
was  covered  by  a  great  shallow  sea  which  ex- 
tended north  to  Canada,  northeast  to  New  York 
State,  and  east  to  the  Allegheny  Mountains.  This  sea  had 
great  islands ;  one  extended  north  from  where  Cincinnati, 
Ohio,  now  stands;  another  was  in  central  Tennessee  about 
where  Nashville  now  exists.  We  know  from  the  fossil  evi- 
dence that  this  sea  did  not  extend  west  of  the  Mississippi  River 
except  in  southern  Missouri.  To  the  south  it  must  have  had 
connection  with  the  open  ocean. 

At  the  end  of  Middle  Devonian  time,   it  is  known  that 

great  crustal  disturbances  took  place  in  southeastern  Missouri. 

Great  faults  were  formed  with  a  maximum  dislocation  of  the 

Crustal  beds    of    1000    feet.      There    is    no    evidence    to 

disturbances       i  ,  .     .1         ,  £  .         ,.  .      , 

in  Devonian      snow   that   any   similar   detormative   disturbance 

times  took  place  in  southeastern  Illinois,  but  the  great 

earthquake    tremors    which    the    Missouri    disturbances    must 

have    created    certainly    affected    this    portion    of    Illinois.      In 

southeastern    Missouri    the   earth    movements   left    the    ancient 

sea  bottom  well   above   sea  level.      It  is   not  improbable   that 

southern    Illinois    was    also    dry    land    for    a    time,    although 

it  was  certainly  again  submerged  in  late  Devonian  time  when 


22  GEOLOGIC    MAKING    OF    SOUTHERN    ILLINOIS 

it  became  a  part  of  a  widespread  interior  sea  which  extended 
from  the  Appalachian  Mountains  on  the  east  to  Oklahoma  on 
the  west,  and  as  far  north  as  Lake  Erie  at  least.  It  extended  as 
far  southeast  as  Chattanooga,  Tennessee,  but  its  southern  ex- 
tent is  not  known  definitely,  for  the  sediments  deposited  are 
hidden  under  much  younger  rocks. 

The  character  of  the  late  Devonian  deposits  indicates  that 
the  waters  of  this  interior  sea  were  more  or  less  stagnant.  The 
sediments  were  fine  muds  with  a  large  admixture    of  organic 

_,  c     matter  which   gave   the  consequent   shales   their 

Character  of      .  B  /*. 

deposits  in  intensely  black  color,  vjuantities  or  the  minute 
Devonian  sea  Sp0re  cases  of  an  ancient  type  of  plant  were 
buried  in  these  muds  and  are  now  preserved  in  the  fossil  con- 
dition. Animal  fossils  are  scarce,  and  those  which  are  present 
show  that  the  sea  was  not  a  typical  body  of  marine  water  and 
that  it  had  only  indirect  communication  with  the  ocean  water 
surrounding  the  continent. 

This  great  interior  sea  with  its  more  or  less  stagnant 
waters  must  have  existed  a  long  time,  humanly  speaking,  for 
no  less  than  400  feet  of  the  characteristic  black  shales  accumu- 
lated, at  least  locally.  The  period,  however,  was  brief  as  geologic 
time  is  reckoned,  and  at  its  close  the  whole  of  southern  Illinois 
was  elevated  above  sea  level.  Once  again  began  the  wearing 
down  processes  of  erosion  which  are  always  effective  when  a 
land  surface  is  elevated  and  whose  vigor  depends  on  the  climate, 
the  elevation  above  sea  level,  and  the  distance  from  the  sea. 

At  last  the  region  was  submerged  again  as  a  result  of 
crustal  deformation  or  of  a  rise  in  sea  level  due  to  changes  else- 
where, and  earth  history  passed  from  the  Devonian  into  the 
Mississippian  Mississippian  period.  The  Mississippian  sea 
sea  which  covered  southern  Illinois  during  this  time 

was  a  widespread  body  of  water  over  the  interior  of  the  con- 
tinent.    It  spread  northward  in  the  present  Mississippi  Valley 


MISSISSIPPIAN    SEA  23 

at  least  as  far  as  Iowa  and  northern  Illinois  and  doubtless  at 
times  it  extended  into  southern  Wisconsin.  At  the  time  of  its 
greatest  extent  it  spread  as  far  to  the  northwest  as  Montana 
and  as  far  to  the  southwest  as  New  Mexico.  To  the  south- 
east it  reached  Alabama  and  Georgia  but  on  the  east  it  was 
limited  by  a  great  land  mass  known  as  Appalachia  which 
occupied  a  position  east  of  the  present  Appalachian  Mountains. 
The  Mississippian  sea,  no  doubt,  had  free  communication  with 
the  open  ocean  to  the  south. 

Conditions  in  the  Mississippian  sea  were  vastly  different, 
however,  from  those  which  gave  rise  to  the  great  accumulation 
of  Devonian  shales,  for  in  it  were  deposited  mainly  limestones 
whose  fossil  contents  consist  of  typical  marine  organisms. 
The  oldest  formations  deposited  in  this  Mississippian  sea  are 
the  limestone  formations  which  encircle  Hicks  Dome.  Younger 
rocks  are  exhibited  in  the  limestone  bluffs  of  Ohio  River  which 
are  more  or  less  continuous  from  Rosiclare  to  Cave-in-Rock, 
in  Hardin  County.  The  same  limestone  formations  are  found 
in  that  part  of  the  county  where  sink  holes  are  so  numerous. 

There    are    several    distinct    limestone    formations    in    the 

Lower  Mississippian.  These  have  been  named  the  Iowan  series 

because  they  are  well  developed  and  were  first  described  in  Iowa. 

They  are  distinguished   by  the  character  of  the 

Mississippian  stra t*i  themselves  and  bv  the  differences  in  fossil 
limestones 

content.  1  he  changes  reflected  by  the  forma- 
tions were  caused  by  changes  in  the  boundaries  of  the  sea, 
in  the  depths  of  the  water,  and  in  the  purity  of  the  water. 
The  purer  limestones  consist  largely  of  calcium  carbonate 
which  was  withdrawn  from  solution  in  the  sea  waters  through 
the  agency  of  shell-forming  animals.  Some  of  the  shells  were 
broken  and  even  ground  into  fine  powder  by  the  move- 
ments   of    the    water    and    formed    the    lime    muds    of    the    sea 


24  GEOLOGIC    MAKING   OF    SOUTHERN    ILLINOIS 

bottom;  others  were  buried  with  little  or  no  injury  in  these 
lime  muds.  Doubtless  many  bacteria  were  present  which  caused 
the  precipitation  of  lime. 

During  part  of  the  time,  the  Ozark  region  of  Missouri 

constituted  a  great  island,  Ozarkia,  which  furnished  materials 

to  the  sea  to  form  shales,   sandstones,   and   impure   limestones. 

Then   again   the   island   was  entirely  submerged 

and,   since  the  source  of  land-derived  sediments 

was  removed  purer  limestones  accumulated. 

Cincinnatia  was  another  great,  low-lying  island,  which 
extended  north  from  Cincinnati,  Ohio.  East  of  it  was  an  arm 
of  the  sea,  which  extended  into  Michigan  and  in  which  little 

limestone   formed,   since  a  continuous  supply  of 

Cincinnatia  i  .  i  .  ,     ,  ,       A 

non-calcareous  material  was  turnisned  by  Appa- 

lachia.  The  shorelines  of  this  great  interior  sea  shifted  con- 
tinually by  reason  of  the  crustal  movements  of  the  earth,  but 
there  is  no  evidence  that  southern  Illinois  was  raised  above  sea 
level  before  the  close  of  the  period. 

The  Iowan  sea  teemed  with  a  great  variety  of  life  as  to 

form,  but  instead  of  the  clams  and  snails  which  make  up  so 

great  a  part  of  the  fauna  of  our  present  oceanic  waters,  there 

were  great  numbers  of  brachiopods  and  crinoids, 

Life  in  the  types  of  life  which  are  met  with  onlv  rarely  in 
Iowan  sea  J  ' 

the  seas  today.     There  were   also  a   few  trilo- 

bites,  a  life  form  which  is  now  wholly  extinct,  some  snails, 
clams,  and  corals.  (See  fig.  6.)  All  these  forms  were 
creatures  which  formed  hard,  stony  shells  which  were  cap- 
able of  fossilization.  There  were  great  numbers  of  fishes  also, 
all  of  them  related  to  the  sharks.  Some  must  have  grown  to  a 
large  size;  their  sole  records  are  their  fossilized  teeth  and  fin 
spines.  There  were  doubtless  also  many  soft-bodied  animals,  as 
there  are  in  the  seas  today,  which  were  wholly  incapable  of 
leaving  records  of  their  existence,  as  well  as  sea  weeds  of  va- 
rious sorts, 


LIFE  IN  THE  IOWAN  SEA 


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26  GEOLOGIC  MAKING  OF  SOUTHERN  ILLINOIS 

Following  the  long  period  of  limestone  formation,  southern 
Illinois  was  raised  above  sea  level;  indeed  it  is  not  unlikely  that 
the  whole  of  the  interior  of  the  continent  became  dry  land. 
Processes  of  erosion  became  active  but  the  low  relief  of  the 
land  prevented  rapid  erosion  and  no  great  thickness  of  the 
limestone  was  removed.  The  emergent  condition  continued 
for  a  considerable  length  of  time  but  not  so  long  that  some 
forms  of  animal  life  which  had  inhabited  the  Iowan  sea  sur- 
vived elsewhere  and  re-entered  the  area  with  a  new  period  of 
submergence. 

The    new    period    of    submergence    in    southern    Illinois 

brought  with   it  a  new  set  of  conditions.     The  rock-forming 

materials  were  no  longer  almost  pure  lime,  but  included  also 

great    quantities    of    sand    and    fine    mud    which 

Chester  built    up    the    Chester    series.      At    the    present 

embayment  ^  .  .  [ 

time  the  Chester  formations  constitute  the  sur- 
face rocks  upon  much  of  the  southern  slope  of  the  Ozark  hills 
in  southern  Illinois  and  in  places  they  continue  to  the  Ohio 
River  bluffs  and  even  across  the  river  into  Kentucky.  The  basin 
in  which  this  series  of  rocks  accumulated  was  very  different 
from  the  great  interior  sea  in  which  the  limestones  of  the  Iowan 
series  were  formed.  In  Illinois  the  basin  wTas  simply  a  great 
embayment  which  extended  northward  between  Cincinnatia  on 
the  east  and  Ozarkia  on  the  west,  to  about  the  middle  of  the 
State.  Into  this  embayment  emptied  great  rivers  whose  sources 
doubtless  were  far  to  the  north  in  what  we  now  know  as  Can- 
ada. These  rivers  brought  down  much  sand  and  fine  mud  which 
they  dumped  into  the  Chester  embayment.  The  waves  along  the 
shores  sorted  the  materials  and  deposited  them  in  different 
places;  the  sands,  which  represented  the  coarser  materials,  settled 
near  shore ;  the  finer  materials  or  muds  were  held  in  suspension, 
were  carried  out  to  sea,  and  finally  settled  in  the  deeper  waters 
awav  from  the  shores,  to  form  the  mud  beds  from  which  the 


POST-CHESTER    VEGETATION  21 

shales  now  conspicuous  in  the  Chester  series  were  developed.  In 
places  in  the  deeper  waters,  conditions  were  favorable  for  the 
existence  of  a  wonderful  variety  of  living  animals,  whose  re- 
mains were  buried  in  the  limy  shale  beds  where  their  hard  parts 
were  preserved  and  from  which  an  abundance  of  fossils  may 
now  be  collected.  In  other  places  in  this  Chester  sea,  conditions 
permitted  the  accumulation  of  pure  limestones  which  can  now 
be  seen  in  many  of  the  exposures  of  Chester  rocks. 

Had  the  shoreline  of  the  Chester  embayment  remained 
stationary  throughout  Chester  time,  there  would  have  been 
deposited    one   great   sandstone    formation   near   shore,    which 

would   have   become   finer   and   finer   off   shore, 
The  shifting     merging  into  a  shale  formation  composed  of  the 

fine  muds  which  had  been  held  in  suspension,  and 
still  farther  out  from  shore  into  a  limestone.  But  such  simple 
conditions  did  not  prevail.  The  shores  of  the  embayment  shifted 
constantly,  and  the  areas  which  at  one  time  were  submerged 
became  exposed  because  of  the  rising  of  the  land  or  the  lowering 
of  the  sea  level — only  to  be  covered  again  by  the  sea  because  of 
a  reversal  of  conditions.  At  intervals  the  whole  of  southern 
Illinois  was  above  water,  and  the  shoreline  lay  to  the  south 
perhaps  across  what  is  now  Kentucky.  As  a  result  of  these 
shiftings  in  the  position  of  the  shoreline,  that  portion  of  southern 
Illinois  where  Chester  rocks  form  the  surface  was  successively 
within  the  zones  of  sandstone,  shale,  and  limestone  formation, 
and  so  the  Chester  strata  are  an  alternating  series  of  these  dif- 
ferent rocks.  The  Chester  section  gives  evidence  of  no  less  than 
eight  shiftings  in  the  shoreline. 

After  the  final  withdrawal  of  the  Chester  sea,  the  whole 
of  the  southern  portion  of  North  America  doubtless  became 
dry  land  and  remained  as  dry  land  for  a  long  time.  A  rich 
Vegetation  of  ve£etation  sprang  up,  very  different  from  what 
post-Chester     now   covers   the    region.      Instead   of   the   usual 

types  of  forest  trees  and  herbaceous  and  shrubbj 


28  GEOLOGIC  MAKING  OF  SOUTHERN  ILLINOIS 

plants  with  which  we  are  familiar,  a  large  number  of  the 
trees  and  other  plants  were  fern-like  in  character ;  others 
were  gigantic  relatives  of  the  little  club  mosses  of  the  northern 
portion  of  our  country  which  now  grow  to  a  height  of  only  a 
few  inches  and  are  not  found  at  all  in  southern  Illinois.  Others 
were  great  tree-like  relatives  of  the  horsetails  which  now  grow 
as  slender  plants  only  a  few  feet  high  at  most,  and  still  others 
were  cone-bearing  trees  with  long  and  broad  grass-like  leaves. 
Nowhere  in  all  these  forests  was  there  a  single  representative 
of  the  plants  bearing  showy  and  sweet-scented  flowers  which 
make  our  own  landscapes  so  beautiful. 

The  air-breathing  animals  of  these  forests  were  also  strange 
forms  when  compared  with  those  we  know  today.  Doubtless 
there  were  numerous  forms  related  to  our  living  salamanders, 
A  .  ...-  f  lizard-like  forms  who  spent  their  early  life 
post-Chester  stages  in  the  waters  of  ponds  and  swamps,  much 
times  as    £ots    the    tadpole   which    is    the    early    stage 

of  the  toad  or  frog.  Perhaps  there  were  a  few  air-breath- 
ing reptiles,  lizard-like  creatures,  which  lived  on  land. 
There  may  have  been  some  land  snails  and  fresh  water  snails 
and  clams.  Flying  about  in  these  forests  were  many  extraord- 
inarily large  insects,  of  the  cockroach  and  dragonfly  type  for 
the  most  part.  One  dragonfly  with  a  wing  spread  of  thirty 
inches  is  known,  and  some  of  the  cockroaches  which  lived  dur- 
ing this  period  were  more  than  a  foot  long. 

This  period  was  the  beginning  of  what  we  call  Pennsyl- 
vanian  time.  Dry  land  then  prevailed  in  this  section  of  the 
continent,  but  to  the  east — along  the  border  of  old  Appalachia 
from  Pennsylvania  southward  to  Georgia — great 
beginning*  of  quantities  of  terrestrial  deposits  of  coarse  sands 
Pennsyl-  and  conglomerates  and  some  coal  were  accumu- 

lating. To  the  southwest,  in  Oklahoma  and  be- 
yond, aquatic  sediments,  in  part  marine,  were  being  deposited. 


ORIGIN    OF   THE    ILLINOIS    COAL   BASIN  29 

How  long  this  dry-land  condition  continued  in  Illinois 
cannot  be  measured  in  years.  Popularly  speaking,  the  time  was 
long,  and  even  from  the  standpoint  of  geology  it  was  a  notable 

interval.  During  this  time,  the  land  surface  was 
Pottsville  undergoing  slow  erosion ;  since  the  region  was  not 

highly  elevated  above  sea  level,  the  erosion  was 
not  as  vigorous  as  in  more  mountainous  regions.  Large  river 
systems  were  developed,  and  some  hundreds  of  feet  of  the  older 
sediments  were  removed. 

Later  a  sinking  of  the  land  resulted  in  a  great  wash  of 
terrestrial  sediments  overspreading  the  region.  These  consisted 
of  coarse  sands  and  conglomerates,  which  filled  the  depressions 
in  the  land  surface,  and  some  finer  sands  and  sandy  shales.  In 
some  places  conditions  were  favorable  for  the  accumulation  of 
vegetable  matter  in  swamps;  the  peat  wThich  was  then  formed 
later  became  coal. 

These  sediments  furnish  the  record  of  events  in  the  Potts- 
ville epoch  of  early  Pennsylvanian  time,  and  their  remnants  now 
form  the  crest  of  the  so-called  Ozark  ridge  which  extends  east 
and  west  across  the  State  south  of  Harrisburg  (fig.  1).  At 
Harrisburg,  these  same  beds  are  far  beneath  the  surface,  because 
of  their  northward  dip  brought  about  by  later  deformation  in 
the  region,  but  they  were  originally  laid  down  in  an  essentially 
horizontal  position. 

After   a   time,   the   region   which    extended    south    beyond 

Ohio  River,  north  into  northern  Illinois,  and  west  across  Iowa, 

Missouri,    and    Arkansas    to    Kansas    and    Oklahoma,    became 

^  .  .       f         stabilized  at  a  level  not  much  higher  than  that 
Origin  of 

Illinois  coal       of  the  ocean.      Upon  this  low,   flat  expanse  the 
basin  drainage  became  more  or  less  stagnant  and  great 

swamps  came   into  existence    (fig.   7),   some  of  them  of  great 
extent,   in  which  the  abundant  remains  of  vegetation   slowly 


30 


GEOLOGIC    MAKING    OF    SOUTHERN    ILLINOIS 


collected.  The  conditions  were  such  that  this  vegetable 
matter  did  not  decay  readily,  and  it  accumulated  first  as 
great  beds  of  peat.  Occasionally  quantities  of  sand  and 
mud  were  washed  into  the  basin  probably  as  a  result  of  slight 
elevations  of  lands  to  the  north,  and  the  peat  beds  were  thus 
buried  under  strata  of  sandstone  and  shale.  At  other  times, 
marine   waters   came   in   and   spread   over   the   area   and   lime- 


Fig.    7. — Reconstruction   of    swamp   forest   in    Pennsylvanian   time. 
After  H.  Potonie. 

stone  beds,  some  nearly  pure  and  others  more  or  less  impure, 
all  carrying  characteristic  marine  shells,  were  formed  in  the 
basin.  Again,  muds  were  deposited  under  marine  conditions,  a 
fact  of  which  the  presence  of  marine  shells  is  an  evidence.  Such 
conditions  were  followed  by  a  recurrence  of  swamps  in  which 
more  peat  beds  were  formed. 

The  result  of  these  successive  changes  wTas  the  building  up 
of  a  series  of  sandstones,   shales,   coals,   and   limestones  which 


ILLINOIS    OZARKS  31 

constitute  the  section  now  found  in  the  Illinois  coal  basin.  The 
great  weight  and  pressure  of  the  superimposed  strata  upon 
the  peat  beds  caused  these  beds  to  be  transformed  gradually  into 
the  coal  which  is  the  important  mineral  resource  of  southern 
Illinois. 

During  this  period  of  accumulation,  the  sediments  were 
laid  down  in  an  approximately  horizontal  position,  and  the  coal 
beds  of  Illinois  and  of  western  Kentucky  were  continuous.  Yet 
today  the  Illinois  fields  are  separated  from  the  Kentucky  fields, 
and  between  them  are  found  rocks  which  are  much  older  than 
the  coal-bearing  strata.  Obviously,  some  deformation  in  the 
earth's  crust  was  responsible  for  the  elevation  of  these  older 
rocks  above  the  coals.  To  the  geologist  comes  the  problem  of 
finding  an  explanation  for  the  condition  which  exists;  why  was 
this  portion  of  the  earth's  crust  deformed,  and  when  did  the 
deformation  take  place? 

A  significant  fact  concerning  the  uplifted  area  in  southern 

Illinois   is   its   position   directly   east   of    the    Ozark   region    of 

Missouri,    which    we    know    was    uplifted    repeatedly    during 

Paleozoic   and    later    geologic    times.      It    is   not 
Relation  of  ,.,    ,       ,  .  ...     ,      T11.      ,  , 

uplifted  area      unlikely  that   the   uplift   in    Illinois   was   due,   in 

to  Missouri  part  at  least,  to  the  proximity  of  the  area  to  the 
^)zarks 

Missouri  Ozarkia  and  its  connection  with  it;  so 

we  may  very  properly  speak  of  the  highlands  of  Illinois  as  a 
continuation  of  the  Ozarks.  That  the  connection  is  popularly 
recognized  is  shown  by  the  fact  that  a  small  town  in  southern 
Illinois  bears  the  name  Ozark. 

The  present  elevation  of  the  Illinois  Ozarks  is  by  no 
means  equal  to  their  original  height.  Perhaps  more  than  a 
thousand  feet  have  been  removed  from  the  tops  of  these  hills 
by  erosion,  and,  although  they  could  never  have  been  really 
high  mountains,  yet  they  must  have  been  a  much  more  formid- 
able range  than  they  are  now  .     Their  present  elevation  is  due  to 


32  GEOLOGIC  MAKING  OF  SOUTHERN  ILLINOIS 

the  facts  not  only  that  the  rocks  were  bowed  up  in  the  past, 
but  also  that  the  older  rocks  which  have  been  uncovered 
are  harder  and  more  resistant  to  erosion  than  the  rocks  to 
the  north.  Consequently  they  remain  as  high  parts  simply  be- 
cause they  are  not  easily  worn  away.  Had  they  been  softer 
than  the  younger  rocks,  a  depression  or  valley  instead  of  a  range 
of  rugged  hills  would  mark  the  belt  of  the  uplift. 

In  the  eastern  portion  of  the  elevated  belt  across  southern 
Illinois,  other  factors  besides  proximity  to  the  Missouri  Ozarks 
had  their  influence.  One  of  these  was  the  intrusion  of  igneous 
Igneous  material  into  the  earth's  crust  which  caused  the 

intrusion  strata  to  dome  up,  as  in  the  case  of  Hicks  Dome. 

As  already  stated,  we  now  find  limestones  of  middle  Devonian 
age  exposed  at  the  surface  in  the  center  of  the  dome,  and  if  all 
the  strata  which  erosion  has  removed  could  be  replaced,  it 
would  be  found  that  the  doming  was  sufficient  to  form  a 
mountain  which  must  have  towered  above  its  surroundings. 

The  molten  material  was  injected  also  into  cracks  and 
fissures  in  the  existing  strata,  and  formed  thin  dikes  (fig.  8) 
which  have  been  found  in  coal  mines  near  Harrisburg,  and  at 
many  other  localities  between  Harrisburg  and  Princeton,  Ken- 
tucky. These  dikes  are  all  similar  in  character  and  must  be 
connected  at  some  unknown  depth  with  a  great  mass  of  igneous 
rock  which  underlies  the  region.  But  the  depth  of  this  igneous 
rock  mass  may  never  be  known  unless  some  extremely  deep 
drilling  should  be  undertaken  at  a  future  time. 

The    injection    of    the    igneous    material    led    to    another 

change,  namely,  faulting.  The  phenomenon  of  faulting  involves 

the  formation  of  more  or  less  vertical  or  oblique  cracks  in  the 

earth's  crust,  extending  to  unknown  depths,  and 

Faulting  t|ie  sj;ppjng  0f  the  rock  strata  on  either  side  of 

the  cracks  vertically,  or  horizontally,  or  both.  Such  dislo- 
cations in  the  rock  strata  are  known  as  faults.     Tn  all  faults 


FAULTING 


33 


except  where  the  motion  is  entirely  horizontal,  the  rock  strata 
on  one  side  of  a  fault  are  raised  relative  to  the  beds  on  the 
opposite  side ;  the  side  which  is  raised  is  known  as  the  upthrow 
side,  and  the  opposite  as  the  downthrow  side  of  the  fault. 
Faulting  may  be  produced  by  the  exertion  of  a  tension  or 
stretching  force  in  the  earth's  crust.  When  such  a  force  is 
exerted  upon  the  rocks,  many  irregular  cracks  are  likely  to  be 

Dike 


1  -|||||^L 

KrWJf  -            1 

. 

\M 

MUhRmK^H 

Fig.  8.     Dike  in  a  mine  in  Saline  County. 


formed,  the  major  ones  of  which  will  have  a  general  direction 
at  right  angles  to  the  direction  of  the  tension  force.  The  rocks 
will  give  way  in  the  weakest  place,  which  may  be  a  belt  of 
considerable  width,  in  which  the  cracks  may  outline  a  regular 
mosaic  of  blocks  of  various  sizes  and  shapes.  The  blocks  so 
formed  will  slip  downward  and  may  tip  upon  each  other  and 


34 


GEOLOGIC   MAKING   OF   SOUTHERN   ILLINOIS 


finally  come  to  rest  when  the  tension  force  has  been  relieved. 
(Fig.  9.)  So  in  this  area,  as  the  beds  were  domed  up,  the 
inelastic  crust  was  forced  to  stretch  and  innumerable  cracks 
formed.  The  wedge-shaped  blocks  which  were  formed  slipped 
and  slid  on  each  other  and  by  their  adjustment  they  took  up 
the  slack  produced  by  the  stretching.  The  readjustments  along 
these  fractures  form  the  complicated  system  of  faults  which 
exists  in  southeastern  Illinois  and  western  Kentucky.  Along 
some  faults  the  dislocation  has  been  as  great  as  2000  feet,  else- 
where only  a  few  feet. 


Hanging  wall 


Foot  wall 


Fig.  9.  Diagram  to  show  the  movement  of  rock  strata  in  normal 
faulting.  The  hanging  wall  moved  down  with  respect  to  the 
foot  wall.  Bed  A  was  continuous  before  the  faulting  took 
place. 


The  phenomena  of  igneous  intrusion  and  faulting  are  be- 
lieved  to  be   responsible   for   the   accumulation   of   the   famous 
fluorspar  deposits  of  Illinois  and  Kentucky.  (Fig.  10.)  Igneous 
rocks  are  known  to  be  a  source  of  fluorine  and 

Fluorspar  most  of  the  important  ore  bodies  in  the  district 

deposits  . 

are  found  in  veins  located  along  tault  lines.     In 

a  few  places  the  fluorspar  is  not  associated  directly  with  a 
fault,  but  it  is  near  some  fault  to  which  it  is  doubtless  geneti- 
cally related. 


II MK    01'    DEFORMATION 


35 


The  determination  of  the  time  when  all  the  deformation 
and  accompanying  phenomena  took  place  is  a  problem  for  the 
geologist.    We  know  certainly  that  it  occurred  after  the  forma- 
tion  of   the    Pennsylvanian    rocks   for    they   are 

Time  of  _  involved    in    the    deformation.      In    the    extreme 

deformation  .  . 

southern  portion  or  Illinois  are  Cretaceous  strata, 

late    Mesozoic   in    age,    which,    so    far    as   we    know,    are   not 

deformed,   but   the   deformed   older   rocks  pass   beneath   them. 

With    these    data,    which    are    all    we    have    at    the    present 

time,    we    can    only    say    that    the    disturbance    took    place 


Fig.   10. — A  fluorspar   mine   in   southern  Illinois. 


after  the  last  of  the  Pennsylvanian  rocks  were  formed  and 
before  the  deposition  of  the  Cretaceous  rocks,  but  this  interval 
represents  millions  of  years.  The  fault  movements  were  prob- 
ably distributed  over  a  long  period  of  time,  for  no  geologist 
believes  that  they  can  be  instantaneous.  It  is  altogether  prob- 
able that  slight  movements  are  still  taking  place  along  some  of 
the  fault  lines.  Scarcely  a  year  passes  without  the  recording 
of  one  or  more  slight  earthquake  shocks  in  this  portion  of  Ohio 


36  GEOLOGIC    MAKING    OF    SOUTHERN    ILLINOIS 

Valley,  which  are  more  than  likely  the  results  of  settlement 
along  some  of  the  previous  lines  of  deformation.  A  little  more 
than  100  years  ago  a  severe  earthquake  occurred  in  the  middle 
Mississippi  Valley  which  may  have  been  caused  by  an  adjust- 
ment of  greater  magnitude  than  usual  along  one  of  the  frac- 
ture lines.  It  may  be  asserted  with  certainty  that  during  the 
active  period  of  all  this  deformation,  frequent  earthquakes  of 
great  severity  shook  southern  Illinois. 

It  is  not  at  all  unlikely  that  a  preliminary  uplift  of  the 
whole  Illinois  Ozark  region,  associated  with  adjustments  in  the 
Missouri  Ozarkia  and  not  accompanied  by  notable  faulting, 
occurred  long  before  the  igneous  intrusion  and  complex  faulting 
in  the  eastern  portion  of  the  region.  All  of  the  facts  are  not  yet 
known,  but  every  season  of  observation  adds  to  our  under- 
standing. 

There   is  no   evidence   that   the   sea   ever   entered    Illinois 

north  of  the  uplift  after  the  deformation.    The  region  remained 

as    an    extended    land    area    for    an    inconceivably    long    time. 

,    Throughout  Mesozoic  time  it  doubtless  was  the 
Mesozoic  and 
Cenozoic  home  of  successive  generations  of  gigantic   rep- 

conditions  tjies  wnjcn  are  known  to  have  inhabited  North 
America  during  this  time  (fig.  11),  and  these  creatures  were 
doubtless  followed  by  the  great  mammalian  hordes  that  charac- 
terized Cenozoic  time.  Through  these  millions  of  years,  the 
vegetation  of  the  region  also  underwent  profound  changes,  but 
none  of  the  record  is  preserved.  We  know  little  of  the  topo- 
graphic features  of  the  land  during  this  time — whether  it  was  a 
plains  or  hill  country — and  little  of  its  drainage;  but  it  is  likely 
that  the  relief  was  much  greater  than  that  of  our  Illinois 
prairies. 

In  Cretaceous  time,  an  arm  of  the  sea  did  reach  into  the 
southern  tip  of  Illinois  from  the  Gulf  of  Mexico,  but  the  ab- 
sence of  any  typical  marine  fossils  in  the  beds  may  mean  that 


MESOZOIC    AND    CENOZOIC    TIME 


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38  GEOLOGIC  MAKING  OF  SOUTHERN  ILLINOIS 

typical  marine  conditions  never  existed.    There  are  also  widely 

distributed  deposits  in  the  south,  probably  Tertiary  in  age,  but 

it  is  not  likely  that  they  are  marine.     They  were  formed  when 

topographic  conditions  were  far  different  from  those  of  today, 

long  before  Ohio  River  and  its  tributaries  were  a  part  of  the 

landscape. 

With  the  coming  of  the  Pleistocene  time  and  the  great  ice 

age,  glaciers  came  down  from  the  north,  one  of  which  reached 

the  vicinity  of  Harrisburg.     These  destroyed  or  drove  out  all 

animal    and    vegetable    life,    and    their    deposits 

Pleistocene       filled   the   depressions   in   the  surface,   leaving   it 
time  F  . 

approximately  as  we  know  it  today. 

We  look  backward  upon  the  many  millions  of  years  during 
which  all  of  these  events  have  come  about.  When  we  realize 
that  the  entrance  of  man  into  the  story  is  the  very  latest  event 
of  all,  we  can  only  be  amazed  and  overwhelmed  by  a  glance 
into  the  future.  There  is  every  reason  to  predict  that  the  con- 
tinuation of  this  history  into  the  future  may  be  as  long  as  the 
time  which  has  passed.  Man  is  a  mere  infant;  he  has  taken 
only  the  first  step  in  his  career,  but  he  has  learned  to  control 
his  environment  as  no  other  animal  has  ever  done.  The  law  of 
progress  is  manifest,  and  we  cannot  help  feeling  that  in  the 
distant  future  our  descendants  will  look  back  upon  us  in  much 
the  same  manner  as  we  now  look  back  upon  the  men  of  the 
Old  Stone  age. 


GLOSSARY 

Brachiopods — Marine,  invertebrate  shell-animals,  like  those  com- 
monly known  as  "lamp  shells".  They  have  two  "shells"  or  valves 
which  are  different,  one  from  the  other,  but  are  bilaterally 
symmetrical.     See  fig.  6,  p.  25. 

Cephalopod — Marine,  single  shelled  invertebrate  animal,  whose  shell 
may  be  either  straight  or  more  or  less  coiled  in  one  plane  and 
consists  of  several  separate  chambers.  The  pearly  nautilus  is  a 
living  example  of  this  type  of  animal.     See  fig.  6,  p.  25. 

Conglomerate — A  rock  formed  by  the  cementation  of  gravel. 

Crinoid — Marine  animal  sometimes  called  "sea  lily".  It  is  attached 
to  the  sea  floor  by  a  stalk  and  its  upturned  mouth  is  surrounded 
by  long,  feathery  arms.     See  fig.  6,  p.  25. 

Crustal — Pertaining  to  the  outer  portion  of  the  earth,  or  that  portion 
whose  nature  is  partly  known  from  geological  examinations. 

Deformation — Warping,  folding  or  any  massive  displacement  of  the 
earth's  crust. 

Delta — Deposit  of  sediment  at  the  mouth  of  a  river. 

Dike — A  body  of  rock  formed  by  the  solidification  of  fluid  rock 
which  was  forced  into  a  fissure. 

Embayment — A  considerable  portion  of  shallow  ocean  water  upon  the 
continent. 

Emergence — A  change  in  the  relations  of  land  and  ocean  resulting  in 
the  conversion  of  the  ocean  bed  into  land. 

Era — A  major  subdivision  of  geologic  time. 

Erosion — Wearing  away.  Erosion  is  the  term  applied  to  all  the 
processes  by  which  earthy  matter  or  rock  is  loosened  and  re- 
moved from  one  place  to  another. 

Evolution — A  term  usually  applied  to  those  changes  which  animal 
and  plant  life  have  undergone  in  the  history  of  the  earth,  result- 
ing in  the  origin  of  new  and  varied  forms. 

Fault — A  displacement  along  a  fracture  interrupting  the  original 
continuity  of  a  rock  formation. 

Fauna — A  natural  assemblage  of  animals  inhabiting  a  given  area,  or 
existing  within  a  certain  period. 


39 


40  GLOSSARY— Continued 

Fluorspar — A    mineral    composed    of    calcium    and    fluorine,    usually 

transparent  and  purplish  or  pinkish  in  color. 
Fossil — Any  remains,   impression,   or  trace  of  an  animal   or  plant  of 

past  geological  times  which  have  been  preserved  in  rocks. 
Gastropod — A    single-shelled    invertebrate    animal,    whose    shell,    if 

coiled,  is  conical  in  shape.     Snails  and  slugs  are  living  examples. 

See  fig.  6,  p.  25. 
Glacier — A  field  of  ice  of  such  thickness  that  it  moves  outward  from 

the  center  of  accumulation. 
Historical  time — The  time  covered  by  human  history  as  recorded  by 

actual  inscriptions  and  writings. 
Intrusion — A  mass  of  igneous  rock  which,  while  molten,  was  forced 

into  or  between  other  rocks. 
Invertebrate — An  animal  which  does  not  possess  a  spinal  column. 
Larval — Pertaining   to   an   early    stage    in   the   development    of    some 

animals.     The  caterpillar  or  worm  which  eventually  becomes   a 

butterfly  is  a  typical  example. 
Limestone — A  sedimentary  rock  composed  of  consolidated  lime  mud 

or  fossil  shells,  or  both. 
Noachian  Deluge — The  great  flood  of  biblical  fame. 
Outcrop — Exposure  of   rock. 

Peat — Accumulation  of  vegetal  material  under  swamp  conditions. 
Pelecypods — Two-valved     shell     animals,     such     as     clams,     scallops, 

oysters,  and  mussels.     The  two  shells  are  mutually  symmetrical, 

but  there  is  no  symmetry  in  either  shell.     See  fig.  6,  p.  25. 
Period— A  major  sub-division  of  a  geologic  era. 
Relief    (topographic  relief)— The  difference  in  elevation  between  the 

highest  and  lowest  points  of  an  area. 
Residuum — Final  products  of  rock  decomposition  in  place. 
Sandstone — A  rock  formed  by  the  cementation  of  sand  grains. 
Sedimentary   rocks — Rocks   formed  by  the  accumulation   of   sediment 

either  in  water  or  in  air. 
Shale — A   rock   formed   by   the    compacting    or   cementation   of    clay 

or  mud. 
Spore — The    reproductive    body    of    flowerless    plants.      Spores    are 

minute  germs  which  have  the  same  function  as  seeds. 


GLOSSARY— Concluded  41 

Stratigraphic — Pertaining  to  the  order  and  relative  position  of  the 
rocks  of  the  earth's  crust.  For  example,  in  Saline  County  the 
Harrisburg  coal  bed  lies  stratigraphically  below  the  Herrin  coal 
bed. 

Stratum — A  layer  of  rock. 

Structure — The  position  or  attitude  of  rock  strata  with  reference  to  a 
horizontal  plane.  For  example,  rock  strata  have  a  horizontal 
structure  if  they  lie  horizontally. 

Submergence — A  change  in  the  relation  of  land  and  ocean  whereby 
a  former  land  area  is  covered  by  the  sea. 

Surfkial  deposits — Unconsolidated  material  lying  on  bedrock. 

Trilobite — An  extinct  marine  animal  which  resembles  the  crabs, 
crayfish,  and  lobsters.     See  fig.  6,  p.  25. 


